I believe the hollow circle being used as the symbol for zero originates with Arabic where the number of angles in the shape indicated what number was represented but I'm tired as hell so I'm not sure.
Wikipedia says this "The number zero does not have its own Roman numeral, but the word nulla (the Latin word meaning "none") was used by medieval scholars in lieu of 0. Dionysius Exiguus was known to use nulla alongside Roman numerals in 525." so yeah, pretty much.
But I think it would be MMXV